Longitudinal Plasmon Wavelength Research Articles

Structurally Engineered Silica Shells on Gold Nanorods for Biomedical Applications

Owing to their unique optical and chemical properties, gold nanorods (AuNRs) are among the most frequently used nanomaterials for biomedical applications, including cancer therapy, imaging, and drug delivery. In particular, the longitudinal dipole plasmon wavelength of AuNRs can be verified from the visible to the near‐infrared (NIR) region, allowing AuNRs to be used as photodynamic/photothermal and imaging contrast agents. At the same time, the silica shell is important as it enhances stability and facilitates the functionalization and biocompatibility of AuNRs, offering numerous advantages in biomedical applications. In this review, silica‐coated AuNRs from a bioapplication perspective are focused. First, the importance of AuNRs for biomedical applications is explained and the purpose of silica coating on AuNRs is discussed. Then, recent studies on the development of silica‐coated AuNRs from a biomedical perspective are reviewed. Subsequently, various strategies for engineering silica coatings and their properties in the biomedical field are reviewed. This review is expected to promote further research on next‐generation silica‐coated AuNRs for biomedical applications.

Plasmon Enhanced Second Harmonic Generation from ZnO Nanofilms on Vertical Au Nanorod Arrays

Vertically aligned gold nanorod arrays have attracted much attention for their fascinating optical properties. Different from longitudinal surface plasmon wavelength (LSPW) and edge-to-edge spacing of gold nanorods, the role of gold nanorod diameter in plasmonic enhancement ability of vertical gold nanorod arrays has rarely been explored. In this work, we selected gold nanorods with similar LSPW but two different diameters (22 and 41 nm), the optical properties of which are dominated by absorption and scattering cross sections, respectively. The vertically aligned arrays of these gold nanorods formed by evaporation self-assembly are coupled with nonlinear ZnO nanocrystal films spin-coated on their surfaces. It was found that the gold nanorod array with a larger diameter can enhance the second harmonic generation (SHG) of ZnO nanofilm by a factor of 27.0, while it is about 7.3 for the smaller gold nanorod array. Theoretical simulations indicate that such stronger enhancement of the larger vertical gold nanorod array compared with the smaller one is due to its stronger scattering ability and greater extent of near-field enhancement at SHG fundamental wavelength. Our work shows that the diameter of gold nanorods is also an important factor to be considered in realizing strong plasmon enhancement of vertically aligned gold nanorod arrays.

Gold Nanobipyramids: An Emerging and Versatile Type of Plasmonic Nanoparticles.

Gold nanobipyramids (Au NBPs) and gold nanorods (Au NRs) are two types of elongated plasmonic nanoparticles with their longitudinal dipolar plasmon wavelengths synthetically tunable from the visible region to the near-infrared region. Both have highly polarization-dependent absorption and scattering cross sections because of their anisotropic geometries. In terms of their differences, each Au NBP has five equally angularly separated twinning planes that are aligned parallel to the length direction, while the most common Au NRs are single-crystalline. As a result, Au NBPs possess two sharp end tips, while Au NRs have rounded or flat ends, resulting in very different plasmonic properties. In general, Au NBPs exhibit larger local electric field enhancements, larger optical cross sections, narrower line widths, better shape and size uniformity, and higher refractive index sensitivity than Au NRs. With the recent development of reliable methods for the growth of Au NBPs with high purity and uniformity, Au NBPs have been attracting much interest for the investigation of their intriguing plasmonic properties and applications. In this Account, we provide a concise introduction to Au NBPs, including their fascinating plasmonic properties, wet-chemistry growth methods, plasmonic applications, and structure-directing function. The synthesis of uniform Au NBPs with variable sizes is of vital importance to control their plasmonic properties. In the synthesis part, we summarize the recent developments on the synthesis of Au NBPs, with a focus on the role of seeds in the seed-mediated growth of pentatwinned Au NBPs and methods to improve their number purity. The excellent plasmonic properties of Au NBPs make them promising candidates for numerous applications. To further explore the largely improved functionalities of Au NBPs, different types of Au-NBP-based hybrid nanostructures have been prepared. They exhibit synergistic interactions between Au NBPs and the other components. We highlight the widespread plasmonic applications of Au NBPs and Au-NBP-based hybrid nanostructures in the fields of spectroscopy, photocatalysis, sensing, switching, and biomedical technologies. We next turn to the structure-directing function of Au NBPs to demonstrate the Au-NBP-directed growth of metal nanostructures and their applications. The structure-directing function is enabled by the unique pentatwinned crystalline structure of Au NBPs. Finally, we conclude with remarks on the future perspectives and research directions on Au NBPs as well as the remaining challenges. We hope that this Account will act as a platform to offer fascinating opportunities and stimulate fast-growing research on the various aspects of Au NBPs.

1-nm-Wide Hydrated Dipole Arrays Regulate AuNW Assembly on Striped Monolayers in Nonpolar Solvent

Summary We show that striped phases of horizontally oriented phospholipids presenting 1-nm-wide orientable dipole arrays can order and straighten flexible 2-nm-diameter gold nanowires (AuNWs) with lengths (up to 1 μm) that greatly exceed the template pitch (∼7.5 nm). AuNW ordering can extend over areas > 100 μm2. Whereas wires bundle in solution, with contact between ligand shells, wires adsorbing to the template separate to much larger center-to-center distances (e.g., 12–30 nm) near multiples of the template pitch, consistent with repulsive interactions between wires emerging during interactions with dipole arrays in the template. AuNW assembly was carried out in cyclohexane, a nonpolar solvent. Interestingly, we found that wire ordering was contingent on the extent of phospholipid headgroup hydration and the presence of excess oleylamine, which assembled into hemicylindrical micelles around the hydrated headgroups. Together, these components generated a protected polar environment that enabled the phospholipid headgroups to function collectively to regulate AuNW assembly.

Synthesis, detection, and modification of the surface of gold nanorods and their optical sensitivity for low concentrations of iron (III) ions

In this paper, a nanosensor has been developed to detect metal Fe3 + ions based on gold nanorods. The gold nanorods used in this research were synthesized using a modified seed-mediated growth method that minimizes the presence of spherical gold nanoparticles in the solution and, with controlled growth of nanorods, uniformly distributes them in the solution. Under reaction conditions, iron metal ions react with gold nanorods, resulting in a change in the structure of the nanoparticles and, consequently, their plasmonic properties. The detection of Fe3+ ions was carried out solely by the absorption mechanism and due to the changes in the longitudinal plasmon wavelength of the gold nanorods. One of the important achievements of this method is the detection limit of water-soluble Fe3+ ions, which is obtained at 0.1 ppm. The properties of gold nanorods and the interaction of Fe3+ ions with nanosensors were studied using a UV–vis spectrophotometer; FT-IR, XRD and transmission electron microscopy (TEM) spectroscopies were used to investigate the morphology and nature of synthetized gold nanorods. In this research, a new nanosensor was introduced to detect water-soluble Fe3+ ions at a concentration of parts per million (ppm), which could eliminate the need for complex chemical sensors and advanced expensive equipment, and provide a simple, low-cost and highly selective detection platform.

Realization of Red Plasmon Shifts up to ∼900 nm by AgPd-Tipping Elongated Au Nanocrystals.

The synthesis of metal nanostructures with plasmon wavelengths beyond ∼1000 nm is strongly desired, especially for those with small sizes. Herein we report on a AgPd-tipping process on Au nanobipyramids with the resultant red plasmon shifts reaching up to ∼900 nm. The large red plasmon shifts are ascribed to the deposition of the metal at the tips of Au nanobipyramids, which is verified by electrodynamic simulations. The method has been successfully applied to Au nanobipyramids and nanorods with different longitudinal dipolar plasmon wavelengths, demonstrating that the plasmon wavelengths of these Au nanocrystals can be extended to the entire near-infrared region. Pt can also induce the tipping on Au nanobipyramids and nanorods to realize red plasmon shifts, suggesting the generality of our approach. We have further shown that the metal-tipped Au nanobipyramids possess a high photothermal conversion efficiency and good photothermal therapy performance. This study opens up a route to the construction of Au nanostructures with plasmon resonance in a broad spectral region for plasmon-enabled technological applications.

Peroxidase-Like Activity of Ethylene Diamine Tetraacetic Acid and Its Application for Ultrasensitive Detection of Tumor Biomarkers and Circular Tumor Cells.

Ethylene diamine tetraacetic acid (EDTA) is such a powerful chelating agent that it may form stable complexes with most metal ions, which has wide applications in industry, agriculture, environment, and pharmaceutical technology. Recently, EDTA was found to enhance the photocatalytic property of some materials. Inspired by this fact of EDTA in the photocatalytic role, we further investigated the photocatalytic property of EDTA and found much the same as that of natural horseradish peroxidase (HRP). This significant discovery of peroxidase-like property may extend the applications of conventional EDTA in life science. A novel and colorimetric sensor based on the peroxidase-like EDTA and unique gold nanorods (GNRs) was designed. Under light irradiation, EDTA may catalyze decomposition of hydrogen peroxide and in situ regulate the longitudinal plasmon wavelength (LPW) of GNRs, displaying various color solution as a read-out means. This colorimetric nanosensor has a great potential to develop into a platform to quantitatively determine analytes as long as the specific antibodies against them were available. Biomarkers of different diseases, such as breast cancer and prostate cancer, were detected with high accuracy. Moreover, combined with immunomagnetic separation of circulating tumor cells (CTCs) from blood, a visual read-out for detection of CTCs was established, which has promising applications in clinical diagnosis, environmental monitoring, and food quality control only using naked eyes.

Cluster Plasmonics: Dielectric and Shape Effects on DNA-Stabilized Silver Clusters.

This work investigates the effects of dielectric environment and cluster shape on electronic excitations of fluorescent DNA-stabilized silver clusters, AgN-DNA. We first establish that the longitudinal plasmon wavelengths predicted by classical Mie-Gans (MG) theory agree with previous quantum calculations for excitation wavelengths of linear silver atom chains, even for clusters of just a few atoms. Application of MG theory to AgN-DNA with 400-850 nm cluster excitation wavelengths indicates that these clusters are characterized by a collective excitation process and suggests effective cluster thicknesses of ∼2 silver atoms and aspect ratios of 1.5 to 5. To investigate sensitivity to the surrounding medium, we measure the wavelength shifts produced by addition of glycerol. These are smaller than reported for much larger gold nanoparticles but easily detectable due to narrower line widths, suggesting that AgN-DNA may have potential for fluorescence-reported changes in dielectric environment at length scales of ∼1 nm.

Production of Monodisperse Gold Nanobipyramids with Number Percentages Approaching 100% and Evaluation of Their Plasmonic Properties

Gold nanobipyramids (NBPs) and nanorods (NRs) are two common types of elongated colloidal plasmonic metal nanocrystals, with their longitudinal plasmon wavelengths synthetically tunable over a wide spectral range. Au NBPs have sharper tips and narrower shape and size distributions than Au NRs. However, the number percentages of Au NBPs have been limited below ≈60%. Herein, a method for producing Au NBPs with number percentages approaching 100% and longitudinal plasmon resonance wavelengths synthetically tuned from ≈700 to ≈1200 nm is reported. This method relies on a stepwise combination of seed‐mediated growth, Ag overgrowth, depletion force‐induced self‐separation, and final chemical etching of Ag. The obtained Au NBPs have the same shapes and sizes as the directly grown ones. Systematic comparisons of the plasmonic properties between the purified Au NBP and high‐yield single‐crystalline Au NR samples show unambiguously that Au NBPs are superior to Au NRs in terms of the plasmon peak width, refractive index sensitivity, figure of merit, two‐photon photoluminescence, and surface‐enhanced Raman scattering.

Multiplex Plasmonic Sensor for Detection of Different Metal Ions Based on a Single Type of Gold Nanorod

In this paper, a label-free multiplex plasmonic sensor has been developed to selectively determine different metal ions including Fe(3+), Hg(2+), Cu(2+), and Ag(+) ions based on a single type of gold nanorod (GNR). Under proper conditions, these metal ions can react with GNRs, resulting in changes of nanostructure and composition. The determination of Fe(3+), Hg(2+), Cu(2+), and Ag(+) ions is therefore readily implemented due to changes of longitudinal plasmon wavelength (LPW) of nanorods. Moreover, the GNR-based assay can not only determine all four kinds of metal ions successively but also can detect which of any one or several kinds of metal ions. This assay is sensitive to detect Fe(3+), Hg(2+), Cu(2+), and Ag(+) as low as 10(-6), 10(-8), 10(-10), and 10(-8) M, respectively. Importantly, the special nanostructure and composition of the nanorods are induced by these metal ions, which allow this sensor to maintain high selectivity to determine these metal ions. This nanosensor abrogates the need for complicated chemosensors or sophisticated equipment, providing a simple and highly selective detection platform.

Coating fabrics with gold nanorods for colouring, UV-protection, and antibacterial functions

Gold nanorods exhibit rich colours owing to the nearly linear dependence of the longitudinal plasmon resonance wavelength on the length-to-diameter aspect ratio. This property of Au nanorods has been utilized in this work for dyeing fabrics. Au nanorods of different aspect ratios were deposited on both cotton and silk fabrics by immersing them in Au nanorod solutions. The coating of Au nanorods makes the fabrics exhibit a broad range of colours varying from brownish red through green to purplish red, which are essentially determined by the longitudinal plasmon wavelength of the deposited Au nanorods. The colorimetric values of the coated fabrics were carefully measured for examining the colouring effects. The nanorod-coated cotton fabrics were found to be commercially acceptable in washing fastness to laundering tests and colour fastness to dry cleaning tests. Moreover, the nanorod-coated cotton and silk fabrics show significant improvements on both UV-protection and antibacterial functions. Our study therefore points out a promising approach for the use of noble metal nanocrystals as dyeing materials for textile applications on the basis of their inherent localized plasmon resonance properties.

Seed-mediated growth and manipulation of Au nanorods via size-controlled synthesis of Au seeds

Seed-mediated growth of gold (Au) nanorods with highly controllable length, width, and aspect ratio was accomplished via carefully size-controlled synthesis of the original Au seeds. A slow dynamic growth of Au nanoparticle seeds was observed after reduction of the Au salt (i.e., hydrogen tetrachloroaurate (III) hydrate) by sodium borohydride (NaBH4) in the presence of cetyltrimethyl ammonium bromide (CTAB). As such, the size of the Au nanoparticle seeds can therefore be manipulated through control over the duration of the reaction period (i.e., aging times of 2, 8, 48, 72, and 144 h were used in this study). These differently sized Au nanoparticles were subsequently used as seeds for the growth of Au nanorods, where the additions of Au salt, CTAB, AgNO3, and ascorbic acid were employed. Smaller Au nanoparticle seeds obtained via short growth/aging time resulted in Au nanorods with higher aspect ratio and thus longer longitudinal surface plasmon wavelength (LSPW). The larger Au nanoparticle seeds obtained via longer growth/aging time resulted in Au nanorods with lower aspect ratio and shorter LSPW.

Electron Structure Modification of Interfaces in SiO2 Glass with PbO Nanoparticles at Gamma-Irradiation

Widely used SiO2-PbO glasses can be a good model for studying electron structure of insulator-metal interface with many Si-O-Pb bonds and eutectic Pb-Si mixture. Gamma-radiation of 60Co with an energy of -1.25 MeV generates efficiently electronic and hole structure defects at these interfaces, which are detected with the resonance optical absorption technique. We studied absorption spectra of polished plates of SiO2 glass with 24% PbO after the irradiation to doses 10(4)-10(6) R at 467 R/s and 300 K. Prior the irradiation there are narrow resonances at 220 nm (oxygen deficient SiO3 centers) and also 285 and 310 nm (transversal and longitudinal plasmons on the rod like Pb-nanoparticles). After irradiation in the above mentioned dose interval the longitudinal plasmon wavelength increases up to 500 nm and that of transversal one decreases down to 260 nm that evidences almost 2-times elongation of the nanoparticles. Respectively, there occurs 10-times growth of the charge carrier density that can ensure a higher value of electric conductivity at the metal-insulator interfaces.

“Ship‐in‐a‐Bottle” Growth of Noble Metal Nanostructures

AbstractNoble metal nanostructures are grown inside hollow mesoporous silica microspheres using “ship‐in‐a‐bottle” growth. Small Au seeds are first introduced into the interior of the hollow microspheres. Au nanorods with synthetically tunable longitudinal plasmon wavelengths and Au nanospheres are obtained through seed‐mediated growth within the microspheres. The encapsulated Au nanocrystals are further coated with Pd or Pt shells. The microsphere‐encapsulated bimetallic core/shell nanostructures can function as catalysts. They exhibit high catalytic performance and their stability is superior to that of the corresponding unencapsulated core/shell nanostructures in the catalytic oxidation of o‐phenylenediamine with hydrogen peroxide. Therefore, these hollow microsphere‐encapsulated metal nanostructures are promising as recoverable and efficient catalysts for various liquid‐phase catalytic reactions.

Plasmon-Controlled Förster Resonance Energy Transfer

The localized plasmons of metal nanocrystals have been widely utilized to control a variety of optical signals, such as Raman, fluorescence, and circular dichroism, from proximal dye molecules. We show, on the single-particle level, that the Forster resonance energy transfer between two different fluorophores can be modulated by adjacent plasmonic nanocrystals. The donor and acceptor fluorophore molecules are embedded in a mesostructured silica shell that is uniformly coated on Au–Ag core–shell nanocrystals. The longitudinal plasmon wavelengths of the core–shell metal nanocrystals are synthetically tailored by varying the aspect ratio. Comparison of the scattering and fluorescence spectra taken from the different hybrid nanostructures indicates that the energy transfer efficiency can be controlled by the plasmon wavelength. When the plasmon peak overlaps with the emission peak of the donor, the energy transfer channel is turned off. When the plasmon peak is red-shifted to be in between the emission peak o...

Preparation and application of novel nanocomposites of magnetic-Au nanorod in SPR biosensor

A novel nanocomposite Fe3O4–Au nanorod (AuNR) was prepared and used as the substrate in the surface plasmon resonance (SPR) biosensor to detect goat IgM. Fe3O4–AuNR nanocomposites were synthesized by a method of seed-mediated growth, and further characterized by molecular absorption spectroscopy, transmission electronic microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The nanocomposites exhibit both magnetic property and exceptional optical property, which are beneficial to the antibody immobilization and the sensitivity of detection. The sensing membrane can be regenerated easily and the experimental procedure is simplified. Moreover, the Au nanorods show two plasmon resonance wavelengths defined as transverse mode and longitudinal mode, and the longitudinal plasmon wavelengths are more sensitive to the changes in the dielectric properties of the surroundings. Fe3O4–AuNR nanocomposites got a high sensitivity in detection of antibody–antigen immunoassay. In the optimal conditions, the biosensor based on Fe3O4–AuNR nanocomposites exhibits a satisfactory response to goat IgM in the concentration range of 0.15–40.00μgmL−1. However, the biosensor without Fe3O4–AuNR nanocomposites shows a response to goat IgM in the concentration range of 1.25–40.00μgmL−1. As a result, the sensitivity of the biosensor based on Fe3O4–AuNR nanocomposites is enhanced significantly.

Refractive Index Sensitivities of Noble Metal Nanocrystals: The Effects of Multipolar Plasmon Resonances and the Metal Type

The refractive index change-based plasmonic sensing with noble metal nanocrystals has been demonstrated unambiguously in a number of studies. The refractive index sensitivity and figure of merit of noble metal nanocrystals are two key parameters for this type of plasmonic sensing. Here, we report our studies of the effects of multipolar plasmon resonances and the metal type on the refractive index sensitivity and figure of merit of noble metal nanocrystals. Ag nanocubes of 84 nm in edge length are found to exhibit well-defined dipolar, quadrupolar, and octupolar resonances. The dipolar resonance has the highest index sensitivity of 336 nm/RIU (refractive index unit), while the quadrupolar resonance possesses the largest figure of merit of 4.55. A comparison between the Ag and Au nanocubes that have comparable particle sizes and dipolar plasmon resonances in a similar spectral region shows that the index sensitivity of the Ag nanocubes is about twice that of the Au nanocubes. The effect of the metal type on the index sensitivity is further confirmed by coating a Ag shell around Au nanorods, where the index sensitivity of the Au−Ag core−shell nanobars is determined to be larger than that of the uncoated Au nanorods that have a longitudinal dipolar plasmon wavelength close to that of the core−shell nanobars. The experimentally observed overall trend in the index sensitivity is also in agreement with that revealed by electrodynamic calculations.

Influence of Gold Nanorod Geometry on Optical Response

As noble metal nanoparticles are deployed into increasingly sophisticated environments, it is necessary to fully develop our understanding of nanoparticle behavior and the corresponding instrument responses. In this paper, we report on the optical response of three important gold nanorod configurations under dark field and differential interference contrast (DIC) microscopy after first establishing their absolute geometries with transmission electron microscopy (TEM). The observed longitudinal plasmon wavelengths of single nanorods are located at wavelengths consistent with previously developed theory. A dimer is shown exhibiting a multipole plasmon at wavelengths that are consistent with the dipole plasmon of single nanorods in the sample. DIC can also distinguish a single nanorod from a pair of uncoupled nanorods with an interparticle distance below the diffraction limit. The experimental observations are consistent with simulated DIC images using a DIC point spread function. The findings herein are a critical step toward being able to characterize nanorods in dynamic environments without the use of electron microscopy.

Plasmon Coupling in Silver Nanosphere Pairs

In this work we study the optical properties of silver metal nanosphere pairs by means of rigorous electrodynamics calculations based on the generalized multiparticle Mie theory. By correlating the dipole longitudinal plasmon wavelength with its corresponding value of the real part of the dielectric constant, and based on the dipole quasi-electrostatic approximation, we were able to derive a new plasmon ruler equation for the longitudinal dipole plasmon resonance of Ag dimers, suitable for a wide range of interparticle distances and radii. The new resonance condition equation, at variance with previous work on noble metal nanoparticle pairs, takes into account empirically retardation effects and multipole interactions, being a novel, accurate, and potentially useful tool for future applications in plasmonic nanometrology or sensor devices.

Strong Polarization Dependence of Plasmon-Enhanced Fluorescence on Single Gold Nanorods

We report on the strong polarization dependence of the plasmon-enhanced fluorescence on single gold nanorods. The fluorescence from the organic fluorophores that are embedded in a mesostructured silica shell around individual gold nanorods is enhanced by the longitudinal plasmon resonance of the nanorods. Our electrostatic calculations show that under an off-resonance excitation, the electric field intensity contour around a nanorod rotates away from the length axis as the excitation polarization is varied. The polarization dependence of the plasmon-enhanced fluorescence is ascribed to the dependence of the averaged electric field intensity enhancement within the silica shell on the excitation polarization. The measured fluorescence enhancement factor is in very good agreement with that obtained from the electrostatic calculations. The fluorescence enhancement factor increases as the longitudinal plasmon wavelength is synthetically tuned close to the excitation wavelength. In addition, the polarization dependence is used to determine the orientation angle of the gold nanorods. The results are in excellent agreement with the actual measurements. Furthermore, the emission spectrum of the fluorophore is modified by the longitudinal plasmon resonance of the gold nanorods. A linear correlation between the emission peak wavelength and the longitudinal plasmon wavelength is obtained.